1. Field of the Invention
The present invention relates to a method of fabricating micro-electromechanical system (MEMS) structures and, more particularly, to a microstructure fabricating method for preventing stiction between a micromachined structure and a substrate or adjacent structures after an etching process for releasing the micromachined structure.
2. Description of the Related Art
As shown in FIG. 1, an MEMS structure commonly referred to as a xe2x80x9cmicrostructurexe2x80x9d is fabricated by wet etching for releasing a portion of a microstructure 106 from a substrate 100 and creating a xe2x80x9csuspendedxe2x80x9d microstructure having a space or gap 110 between a released portion 104 and the substrate 100 and one or more posts 102 attached to the substrate 100. The microstructure 106 including the released portion 104 takes the form of a beam or plate having top and A; bottom surfaces which are suspended to be substantially parallel with the surface of the substrate 100. Examples of a device having the suspended microstructure 106 may include accelerometers, pressure sensors, flow sensors, transducers, and microactuators. The term microstructure in this specification collectively refers to a structure fabricated by an MEMS fabrication technology including but not limited to photolithography, thin film deposition, bulk micromachining, surface micromachining, and etching.
A release etching method has two types of processes: a process to create a cavity in the substrate 100 (xe2x80x9cbulk micromachiningxe2x80x9d) and a process to remove a sacrificial layer at an intermediate portion of the microstructure 106 and the substrate 100 (xe2x80x9csurface micromachiningxe2x80x9d). In both micromachining processes, the released portion 104 of the microstructure 106 often permanently sticks to the substrate 100 or adjacent structures during post-etch cleaning and drying steps as shown in 108 of FIG. 1.
More specifically, in the surface micromachining technique for creating the released portion 104 of the microstructure 106, a sacrificial layer is removed by common wet etching. In this case, the substrate 100 is exposed to a chemical etching solution for decomposing only the sacrificial layer not affecting a material of the microstructure 106 and then cleaned with a cleaning solution. When removing the cleaning solution, the surface tension of the solution affects the released portion 104 of the suspended microstructure 106 to adhere the bottom surface of the released portion 104 to the substrate 100 or adjacent other structures as shown in 108 of FIG. 1. The adhesion is called xe2x80x9cstiction.xe2x80x9d This stiction degrades the sensitivity of a sensor. Furthermore, severe stiction results in failure in device fabrication and consequently drops a micromachining process yield rate.
A variety of techniques have been developed to avoid stiction. One technique is to reduce the contact area between a released portion and a substrate as much as possible. Another technique is to solidify and sublimate an etching cleaning solution which is the cause of the surface tension between the released portion of the microstructure and the substrate. Similar to this technique, as disclosed in G. T. Mulhern et al., Proc. Int. Conf. Solid State Sensors and Actuators (Transducers ""93), Yokohama, 1993 (IEEJ, Tokyo, 1993) p. 296, a pressure-controlled and temperature-controlled chamber is used to sublimate a cleaning solution in its supercritical state.
The above methods for preventing stiction suffer from drops in a yield rate and require additional fabrication steps and expensive equipment. Furthermore, the stiction due to surface tension forces may be basically removed by dry etching, but use of the dry etching may damage the material of a suspended microstructure such as polycrystalline silicon. Furthermore, none of the above methods prevents stiction due to an elastic force other than surface tension.
To solve the above problems, it is an aspect of the present invention to provide a method for fabricating a microstructure that can prevent stiction of a microstructure to a substrate during a microstructure fabrication process in a simple way.
Accordingly, the present invention provides a method of fabricating a micromachined structure suspended above a substrate using a sacrificial layer. The method includes the step of stacking an anti-stiction layer that can be removed by dry etching before or after stacking the sacrificial layer.
The anti-stiction layer, which may be formed of polymer, polycrystalline silicon or photoresist that can be removed by dry etching, is formed on the substrate. However, the anti-stiction layer may be formed after stacking the sacrificial layer.
If the anti-stiction layer is formed on the substrate before stacking the sacrificial layer, the sacrificial layer and the microstructure are sequentially formed on the anti-stiction layer. The sacrificial layer is removed by wet etching. In this case, the anti-stiction layer is removed by dry etching, thus providing a microstructure without stiction.